What Is Ball Lightning?

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SEATTLE - Ball lightning is a very rare form of lightning that really doesn't look like lightning at all.

Instead, it's a glowing ball of light that can range in size from about a grapefruit, up to the size of a small car.

They usually form from the most violent thunderstorms. They are a very strange sight to behold, as these balls of light can either roll along the ground or atop fences, or float thousands of feet in the air -- sort of like electrically charged beach balls. They can either last several seconds, or several minutes.

Usually, they don't cause any damage, but have been known to leave burn marks if they pass through windows or screens. But the most mysterious thing about ball lightning is, to this day, scientists can't explain what causes it.